The attack, which appears to be the first successful terrorist strike in a U.S. city since Sept. 11, 2001, returns security to the center of the public eye.

WASHINGTON — It was the kind of event that had long been predicted — even considered inevitable.

But the explosions Monday in Boston, which appeared to be the first successful terrorist strike against a U.S. city since Sept. 11, struck at the nation's sense of safety in public places and sparked a search for answers.

"In some ways, this ruptures the psyche," said Juan Carlos Zarate, deputy national security advisor in the George W. Bush administration who is now at the Center for Strategic and International Studies in Washington.

The only similar deadly attack on U.S. soil since Sept. 11 was on a military base at Ft. Hood, Texas, and involved soldiers, he noted. Americans have had a "social memory loss" of what it's like to be attacked.

"Now we have that soft target hit that we have imagined but not seen before since 9/11," he said. "We don't know who perpetrated it — we'll have to see. But regardless, it shatters the sense of security we've had, especially coming at an event like this."

A senior U.S. counter-terrorism official said no evidence currently linked the bombing to any specific group or individuals, and no intelligence suggested an international connection.

But government analysts leaned toward the theory that the attack was the work of home-grown radicals inspired by Al Qaeda, as opposed to anti-government extremists. They based that hunch in part on the use of multiple bombs in a public place, which has been a signature of Al Qaeda strikes, and on the fact that the target was not connected to a government building.

"My educated guess would be self-radicalized Islamic extremists from the area," the official said, speaking anonymously to discuss intelligence matters.

The Boston attack was less sophisticated than Sept. 11 or the wave of bombings that killed hundreds on London's subways and Madrid's rail system over the past decade.

Indeed, some terrorism experts said they were puzzled by the selection of the Boston Marathon as a target. The race is a major event that attracts runners worldwide, but it lacks the high international symbolism of some other terrorist targets, including Times Square, the Pentagon and the World Trade Center. That choice may also lend credence to the theory that the attacker or attackers had ties to the Boston area.

"Those previous attempts on Times Square and the New York subway, had they succeeded, would have been substantially more lethal than this," said Philip Mudd, a former senior CIA and FBI official and an expert in Al Qaeda operations. "Objectively I would say it leads me to question whether there is substantial overseas involvement. You've got basic devices without an iconic target."

Ever since Sept. 11, officials had warned that sooner or later, some attacker would succeed, a theme they repeated Monday.

"There's not going to be any way to protect the country completely against individual attacks," said Rep. Adam B. Schiff (D-Burbank), who serves on the House intelligence committee. "But I think you'll find when morning comes a determination not to let this change our way of life. I think the American people are tough and determined and will not let this change how we live our lives."

Despite that sentiment, the threat of terrorism has already changed Americans' lives in ways both large and small, including heightened security at airports and a significant expansion of police powers, which critics say has led to an erosion of civil liberties.

Under administrations of two different political parties, law enforcement agencies have succeeded in thwarting numerous plots, now known mostly by nicknames that underscored their ineffectiveness — the shoe bomber, the underwear bomber and the like. As years have gone by without a successful attack, concern about terrorism has slipped down the list of public concern, and support for some anti-terrorism measures has begun to wane.

As of Monday morning, the nation's public life was sharply focused on the domestic issues of immigration, gun control and the budget deficit.

How much that will change will depend on answers yet to come — who is found responsible for the attack, whether any obvious lapses of security precautions helped make it possible, whether obvious warnings were overlooked.

By day's end, however, it was clear that the Boston bombings had suddenly put terrorism back high on the national agenda.